Corrosion and corrosion mitigation have become increasingly important for economic and safety reasons. Based on estimates made in the mid 1990's, overall costs attributable to corrosion account for over $100 billion a year in the United States alone. These costs typically account for only the direct costs of corrosion and do not include the associated indirect costs, such as safety, plant downtime, loss of product, contamination and over-design.
Corrosion is defined as the destructive result between a metal or metal alloy and its environment. Nearly every metallic corrosion process involves the transfer of electronic charge in aqueous solution, and most corrosion reactions take place in the presence of water in either liquid or condensed vapor phases and also in high humidity.
Corrosion is particularly a problem in marine environments, such as shipboard, off-shore drilling rigs, coastal regions and the like, where seawater enhances corrosion reactions due to increased ion transport, pH effects and elevated dissolved oxygen levels that in turn enhance levels of hydrogen ions. Corrosion reactions are further accelerated in marine environments by contaminants, such as chloride ions, present in seawater. Corrosion damage to equipment stored and used in marine environments is a tremendous problem, impacting maintenance costs, availability, repair and reliability.
Equipment stored, for example, onboard a ship or in coastal regions, is often stored in protective storage systems that have proved to be less than optimally effective. At best, such equipment is covered with waterproof tarpaulins, although often, especially for shipboard equipment, it is not stored properly and is directly exposed to a marine environment, which leads to rapid corrosion. Even when equipment is covered by waterproof tarpaulins, seawater still penetrates through and/or around the tarpaulins into the protected spaces where it collects and corrodes the underlying equipment. Also, conventional storage systems can be cumbersome to use and maintain, and are often avoided. As a result, corrosion continues to be a significant and costly problem, requiring many hours of rust removal, painting and repair that lead to premature equipment replacement.
FIG. 1 shows a conventional waterproof cover 20 used to protect metallic objects, such as metallic block 22 shown resting on a surface 24, from moisture, such as rain, sea spray, dew and the like. Cover 20 has an outer surface 26, an inner surface 28 and an area 30 defined by a peripheral edge 32. Cover 20 is shown covering block 22 in a typical manner, wherein a micro-environment, is generally defined by the space enclosed by cover 20. The micro-environment comprises a number of interior regions, such as regions 34, located between cover 20 and block 22.
Generally, prior art covers comprise at least one liquid-impermeable layer made of, for example, a tightly-woven polymer fabric. More complex prior art covers may include one or more additional layers that provide the inner surface with a non-abrasive texture to minimize mechanical damage to the object covered. Other prior art covers are made of vapor-permeable materials, such as expanded polytetrafluoroethylene or the like.
Interior regions 34 generally never have a moisture content less than that of the ambient environment. If the moisture content of the ambient environment rises, the moisture content of regions 34 also rises due to the inflow of moisture (illustrated by arrow 36) through gaps between cover 20 and surface 24 at peripheral edges 32. Eventually, the moisture content of the ambient environment 38 and regions 34 equalize. Once the additional moisture is in the micro-environment, it can become trapped, as illustrated by arrows 40. Moisture levels can quickly become elevated and the air saturated. In such a case, condensation could occur on the block 22. Because the moisture content of interior regions 34 never falls below that of ambient environment 38, prior art covers are not very effective in high moisture environments, such as marine and high-humidity environments. Moreover, once moisture enters the micro-environment, it can take a long time to dissipate, if at all.